Discover Plus

Discovery+ is a separate streaming service by Discovery Networks. It's like CBS All Access and will probably be separate from your cable subscription.

Paramount plus is just going to be a new name for CBS All Access.

More than a name change. Other content will be integrated as well.

Will Xfinity have Discovery plus on X1 when it launches on December 22nd

Isn't Discovery Plus the channels we already pay for in our cable packages?

Discovery Plus launches on January 4, 2021.

In December 2020, Discovery announced the launch of its streaming service, Discovery Plus. It will launch on January 4, 2021. The service will be available across all major platforms.[81]

https://www.theverge.com/2020/12/2/22018581/discovery-streaming-service-launch-date-price-shark-week-hgtv-food-network

Will Xfinity/Comcast be removing the channels covered under Discovery+ from their cable channel line up like they removed TCM?

TCM was not removed. It was just moved to Sports/Entertainment, and has nothing to do with streaming. Since it has no ads, Comcast could not make a deal to add their own ads to make money.

I doubt if there will be any change to the linear Discovery channels any time soon. Discovery+ is just like HBO Max and similar that just squash all the programs into one streaming subscription. I do notice that in some of their graphics showing the channels it will carry, SCIENCE is not one of them.